Minor enhancements, bug fixes, and man-page adjustments.

+ Make `sysrc -x foo' produce error status if foo is unset.
NB: Reported by lme (everything else ``while I'm at it'')
+ Remove mention of SYSRC_VERBOSE from `sysrc --help' output.
NB: False documentation leftover from an ancient precursor.
+ Make `sysrc -qc foo' work the same as `sysrc -ic foo' when foo is unset
NB: For syntax convenience (my fingers know `-q' more than `-i').
+ Update description of `-c' flag in help message/manual.
+ Update description of `-q' flag in help message/manual.
+ Make `sysrc -vc foo' work as documented in the manual
NB: Show message stating whether foo is currently set.
+ Make `sysrc -vc foo=1' work as documented in the manual
NB: Show message stating how value of foo would be changed (if at all).
+ Remove odd usage of parentheses in `-R dir' section of manual.
+ Clarify syntax section of manual w/respect to sysctl(8) similarities.
+ Add new/missing people to the `THANKS TO' section of the manual.

Reported by:	lme
MFC after:	3 days
X-MFC-to:	stable/10, stable/9, ports
Improve timeout(1) man page

Document the exit values and the duration format
Improve wording
Pet mandoc -Tlint
Sort SEE ALSO

Phabric:	https://phabric.freebsd.org/D432
Reviewed by:	wblock
Add libsbuf.so.6 to obsolete libraries since it has been bumped to .so.7