New APIs: getxattr and lgetxattr to get single extended attributes.

New APIs: getxattr and lgetxattr to get single extended attributes.

These APIs are essentially required to work around a problem
with ntfs-3g.  This filesystem (or FUSE?) does not list all
extended attributes of a file when you call listxattr(2).  However
if you know the name of an extended attribute, you can retrieve
it directly using getxattr(2).

The current APIs (getxattrs etc) are simple to use, but they
don't work if we can't list out the extended attributes (ie.
by calling listxattr(2)).

Example using the new APIs on an ntfs-3g filesystem:

><fs> lgetxattr "/Documents and Settings" system.ntfs_attrib | hexdump -C
00000000  16 24 00 00                                       |.$..|
00000004
><fs> lgetxattr "/Documents and Settings" system.ntfs_reparse_data | hexdump -C
00000000  03 00 00 a0 34 00 00 00  00 00 18 00 1a 00 10 00  |....4...........|
00000010  5c 00 3f 00 3f 00 5c 00  43 00 3a 00 5c 00 55 00  |\.?.?.\.C.:.\.U.|
00000020  73 00 65 00 72 00 73 00  00 00 43 00 3a 00 5c 00  |s.e.r.s...C.:.\.|
00000030  55 00 73 00 65 00 72 00  73 00 00 00              |U.s.e.r.s...|
0000003c
><fs> getxattr "/Documents and Settings" system.ntfs_reparse_data | hexdump -C
libguestfs: error: getxattr: getxattr: No such file or directory
><fs> getxattr "/Documents and Settings" system.ntfs_attrib | hexdump -C
libguestfs: error: getxattr: getxattr: No such file or directory
><fs> lgetxattr "/Documents and Settings" system.ntfs_attrib | hexdump -C
00000000  16 24 00 00                                       |.$..|
00000004
><fs> getxattr "/Users" system.ntfs_attrib | hexdump -C
00000000  11 00 00 00                                       |....|
00000004