LU-19062 llapi: add layout pattern string functions

LU-19062 llapi: add layout pattern string functions

Add llapi_lov_pattern_string() to print arbitrary pattern flags to
a string rather than layout2name() which only can print specific
hard-coded combinations of patterns.

Add llapi_lov_string_pattern() to convert layout pattern names to
flags.

Add enum lov_pattern that holds LOV_PATTERN constants, and use it.

Add description of patterns to lfs-getstripe.1.
Restore "-L, --layout" argument listing to lfs-setstripe.1.

Fixes: b6deb420a8 ("LU-17370 utils: simplify lfs help text")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ie21c7c75c685f3a15ac23e83562a12a3ea2540e5