Document PR_CAPBSET_READ and PR_CAPBSET_DROP.

Document PR_CAPBSET_READ and PR_CAPBSET_DROP.
Document PR_GET_TSC and PR_SET_TSC.
Document PR_SET_SECCOMP and PR_GET_SECCOMP.
PR_SET_KEEPCAPS and PR_GET_KEEPCAPS operate on a per-thread
setting, not a per-process setting.
Clarify fork(2) details for PR_SET_PDEATHSIG.
Add description of PR_SET_SECUREBITS and PR_GET_SECUREBITS,
as well as pointer to further info in capabilities(7).
PR_GET_ENDIAN returns endianness info in location pointed to by
arg2 (not as function result, as was implied by previous text).
Expand description of PR_SET_NAME and PR_GET_NAME.
RETURN VALUE: bring up to date for various options.
Various improvements in ERRORS.
Note that PR_SET_TIMING setting of PR_TIMING_TIMESTAMP is not
currently implemented.
Minor changes:
* Clarify wording for PR_GET_UNALIGN, PR_GET_FPEMU, and PR_GET_FPEXC.
* Some reformatting of kernel version information.
* Reorder PR_GET_ENDIAN and PR_SET_ENDIAN entries.