MFC SVN r329188,329334,329353,329914,329995-329996: DTrace Enhancements

r329188: Use tabs in io.d, fix alignment issues, remove extra newlines
r329334: Add errno definitions to /usr/lib/dtrace/errno.d
r329353: Add inline to errno.d for translating int to string
r329914: Updates and enhancements to io.d to aid DTrace scripting
r329995: Updates and enhancements to signal.d to aid DTrace scripting
r329996: Consistent casing for fallback SIGCHLD (s/Unknown/unknown/)

MFC SVN r330559-330560,330672,332865-332867,333513-333519: dwatch(1)

r330559: Introduce dwatch(1) as a tool for making DTrace more useful
r330560: Bump dwatch(1) internal version from 1.0-beta-91 to 1.0
r330672: Fix display of wrong pid from dtrace_sched(4)
r332865: Add `-dev' option to aid debugging of profiles
r332866: Add profile for send(2)/recv(2) syscalls
r332867: Remove the line used to demonstrate `-dev' option
r333513: Bugfix, usage displayed with `-1Q'
r333514: Separate default values so `-[BK] num' don't affect usage
r333515: Simplify info message test
r333516: Export ARGV to profiles loaded via load_profile()
r333517: Allow `-E code' to override profile EVENT_DETAILS
r333518: Expose process for ip/tcp/udp
r333519: Refactor sendrecv profile

Reviewed by: markj, gnn, bdrewery (head; earlier version)
Approved by: re (gjb)
Relnotes: yes
Sponsored by: Smule, Inc.
Differential Revision: https://reviews.freebsd.org/D15418