gettimeofday: change format to a table with sec and usec fields.

* lposix.c (Pgettimeofday): Return a single table with sec and usec
fields, rather than a pair of numbers.
(Ptimeradd, Ptimercmp, Ptimersub): New functions to emulate the
macros timeradd, timercmp and timersub respectively.
gettimeofday: replace C timer macros with lua equivalents.

* lposix.c (Ptimeradd, Ptimercmp, Ptimersub): Remove.
* posix.lua (timeradd, timercmp, timersub): New Lua equivalents
to removed C implementations.
Merge pull request #41 from gvvaughan/pull-request/timer-macros

gettimeofday: change format to a table with sec and usec fields.
build: don't include non-MIT-licensed dummy.c from gnulib (fixes issue #42).
Simplify construction of messages with autoconf variables.
Fix some incorrectly-named references to posix module.
Update bug reporting advice (thanks, Yuri Takhteyev).
build: don't unset LUA_INIT, as it may be needed to set Lua paths.
lposix.c: revert to using MYNAME, as it is different from PACKAGE.
Simplify Lua 5.2 compatibility
Remove documentation of configure options, one of which was obsolete.
Document dependency on bitop library with Lua 5.1 (thanks to
Bernd Eggink).
Add docstrings (using Steve Donovan's ldoc) to lposix.c

The documentation comes from from Steve Donovan and the Alpine Linux
wiki (Natanael Copa).

Reindent Psignal, which was a bit out of order.

Make rpoll use file descriptors, not Lua file objects, so it can be
used with both (via fileno).
bootstrap.conf: no longer require now-defunct `missing' script.
README: Update location of repo and contact details.
README: Add link to online HTML docs.
Bump version to 5.1.21.
Update Github URL.
Merge pull request #45 from wcmaier/fix/github-url

Update Github URL.
configure.ac: enable automake to generate dependencies for rockspec (thanks, Hisham Muhammad).