LU-12521 llapi: add separate fsname and instance API

LU-12521 llapi: add separate fsname and instance API

The llapi_getname() function returns the combined fsname and client
instance as one string, which is fine when using the entire string,
but the output cannot be safely parsed into separate fsname and
instance strings in all cases.

Introduce new llapi_get_fsname() and llapi_get_instance() functions
that return only the fsname and instance strings, since the source
string returned from the kernel can be unambiguously separated before
it is returned in a combined string via llapi_getname().

Fix the lfs_getname() '-n' and '-i' options to use the new routines
rather than parsing the output from llapi_getname().

Add man pages for these functions.

Fixes: 2a4821b836c8 ("LU-12159 utils: improve lfs getname functionality")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iaf5846a0ae147a428f66ec8a1d0251e7e12540e5
Reviewed-on: https://review.whamcloud.com/35451
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>