Stop using EXTRA_DIST in src/Makefile.am.

Automake finds the files in the subdirectories of src/ now that they are
properly declared in the _SOURCES variables. Using EXTRA_DIST would now cause
.o files to be included in the tarball.
Remove texi2html rule in docs/Makefile.

According to Debian, texi2html is deprecated and makeinfo --html should be used
instead. Automake already provides a html target that invokes makeinfo.
Create UNIX socket at the same time as the PID file is created.

The PID file was created before tinc-up was called, but the UNIX socket was
created afterwards, which meant one could not change the UNIX socket's owner or
permissions from the tinc-up script.