'./autobuild.sh' with lcov installed discovered that our
coverage support has been bit-rotting for a while.  This
restores it back to a successful state, although I have
not yet spent any time looking through the resulting files to
look for low-hanging fruit in the unit test coverage front.

* configure.ac: Clear COMPILER_FLAGS at right place.
* Makefile.am (cov): Newer genhtml no longer likes plain -s.
* m4/compiler-flags.m4 (gl_COMPILER_FLAGS): Don't AC_SUBST
COMPILER_FLAGS; it is a shell variable for use in configure only.
* src/Makefile.am (AM_CFLAGS, AM_LDFLAGS): New variables, to make
it easier to provide global flag additions.  Use throughout, to
uniformly apply coverage flags.
* .gitignore: Globally ignore gcov output.
* daemon/.gitignore: Simplify.
* src/.gitignore: Likewise.
* tests/.gitignore: Likewise.