LU-8998 docs: add llapi_ man pages to Makefile.am

LU-8998 docs: add llapi_ man pages to Makefile.am

Reformat lustre/doc/Makefile.am to have one entry per line to
match master, so the merge is more straight forward.

Add the llapi_layout_comp_*.3 and llapi_layout_file_comp_*.3 man
pages to lustre/doc/Makefile.am so they are included into the RPMs.

Replace references to the non-existent "liblustreapi.7" man page
with "lustreapi.7" instead, and add all llapi_* man pages to it.

Remove the long-obsolete llbackup.8 man page.

Reviewed-on: https://review.whamcloud.com/26275

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Change-Id: Ic97ea204f9635668fd5a4448687019f84f5cab07
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>