config: Use "POSIX" instead of "Linux and Solaris"

As requested by Michael [1], this reduces the number of changes
required if BSD or some such is added to the config.

This change leaves a bit of a gap between the platforms listed in
spec.md, since readers have to figure out on their own that the POSIX
properties apply to the linux and solaris platforms (and potentially
other future platforms).  But Michael felt like it's ok to leave that
gap, at least for now [2].

I've used "On POSIX platforms" and similar (instead of "On POSIX")
because, as Tianon points out [3], there is some space between
POSIX-the-spec and platforms which implement it.

[1]: #838 (comment)
[2]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2017/opencontainers.2017-06-01-17.41.log.html#l-60
[3]: #838 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>

config.md

@@ -36,7 +36,7 @@ For all platform-specific configuration values, the scope defined below in the [
 
     * On Windows, `path` MUST be a [volume GUID path][naming-a-volume].
 
-    * On Linux and Solaris, `path` is either an absolute path or a relative path to the bundle.
+    * On POSIX platforms, `path` is either an absolute path or a relative path to the bundle.
         For example, with a bundle at `/to/bundle` and a root filesystem at `/to/bundle/rootfs`, the `path` value can be either `/to/bundle/rootfs` or `rootfs`.
         The value SHOULD be the conventional `rootfs`.
 
@@ -45,7 +45,7 @@ For all platform-specific configuration values, the scope defined below in the [
 * **`readonly`** (bool, OPTIONAL) If true then the root filesystem MUST be read-only inside the container, defaults to false.
     * On Windows, this field MUST be omitted or false.
 
-### Example (POSIX)
+### Example (POSIX platforms)
 
 ```json
 "root": {
@@ -94,9 +94,9 @@ For all platform-specific configuration values, the scope defined below in the [
 ]
 ```
 
-### <a name="configLinuxAndSolarisMounts" />Linux and Solaris Mounts
+### <a name="configPOSIXMounts" />POSIX-platform Mounts
 
-For Linux and Solaris based systems the mounts structure has the following fields:
+For POSIX platforms the `mounts` structure has the following fields:
 
 * **`type`** (string, OPTIONAL) The type of the filesystem to be mounted.
   * Linux: filesystem types supported by the kernel as listed in */proc/filesystems* (e.g., "minix", "ext2", "ext3", "jfs", "xfs", "reiserfs", "msdos", "proc", "nfs", "iso9660").
@@ -195,9 +195,9 @@ For Linux-based systems the process structure supports the following process-spe
 
 The user for the process is a platform-specific structure that allows specific control over which user the process runs as.
 
-#### <a name="configLinuxAndSolarisUser" />Linux and Solaris User
+#### <a name="configPOSIXUser" />POSIX-platform User
 
-For Linux and Solaris based systems the user structure has the following fields:
+For POSIX platforms the `user` structure has the following fields:
 
 * **`uid`** (int, REQUIRED) specifies the user ID in the [container namespace](glossary.md#container-namespace).
 * **`gid`** (int, REQUIRED) specifies the group ID in the [container namespace](glossary.md#container-namespace).
@@ -348,9 +348,9 @@ For Windows based systems the user structure has the following fields:
 }
 ```
 
-## <a name="configHooks" />Linux and Solaris Hooks
+## <a name="configHooks" />POSIX-platform Hooks
 
-For Linux- and Solaris-based systems, the configuration structure supports `hooks` for configuring custom actions related to the [lifecycle](runtime.md#lifecycle) of the container.
+For POSIX platforms, the configuration structure supports `hooks` for configuring custom actions related to the [lifecycle](runtime.md#lifecycle) of the container.
 
 * **`hooks`** (object, OPTIONAL) MAY contain any of the following properties:
     * **`prestart`** (array of objects, OPTIONAL) is an array of [pre-start hooks](#prestart).