updater: introduce and set_metadata and set_metadata_recursive

Introduce two new updater functions:

* set_metadata
* set_metadata_recursive

Long term, these functions are intended to be more flexible replacements
for the following methods:

* set_perm
* set_perm_recursive

Usage:

  set_metadata("filename", "key1", "value1", "key2", "value2", ...)
  set_metadata_recursive("dirname", "key1", "value1", "key2", "value2", ...)

Description:

set_metadata() and set_metadata_recursive() set the attributes on a file/directory
according to the key/value pairs provided. Today, the following keys are
supported:

* uid
* gid
* mode (set_perm_extd only)
* fmode (set_perm_extd_recursive only)
* dmode (set_perm_extd_recursive only)
* selabel
* capabilities

Unknown keys are logged as warnings, but are not fatal errors.

Examples:

* set_metadata("/system/bin/netcfg", "selabel", "u:object_r:system_file:s0");

This sets the SELinux label of /system/bin/netcfg to u:object_r:system_file:s0.
No other changes occur.

* set_metadata("/system/bin/netcfg", "uid", 0, "gid", 3003, "mode", 02750, "selabel", "u:object_r:system_file:s0", "capabilities", 0x0);

This sets /system/bin/netcfg to uid=0, gid=3003, mode=02750,
selinux label=u:object_r:system_file:s0, and clears the capabilities
associated with the file.

* set_metadata_recursive("/system", "uid", 0, "gid", 0, "fmode", 0644, "dmode", 0755, "selabel", "u:object_r:system_file:s0", "capabilities", 0x0);

All files and directories under /system are set to uid=0, gid=0,
and selinux label=u:object_r:system_file:s0. Directories are set to
mode=0755.  Files are set to mode=0644 and all capabilities are cleared.

Bug: 10183961
Bug: 10186213
Bug: 8985290
Change-Id: Ifdcf186a7ed45265511dc493c4036e1ac5e3d0af