coverage: Make the coverage counters catalog program-specific.

coverage: Make the coverage counters catalog program-specific.

Until now, the collection of coverage counters supported by a given OVS
program was not specific to that program.  That means that, for example,
even though ovs-dpctl does not have anything to do with mac_learning, it
still has a coverage counter for it.  This is confusing, at best.

This commit fixes the problem on some systems, in particular on ones that
use GCC and the GNU linker.  It uses the feature of the GNU linker
described in its manual as:

    If an orphaned section's name is representable as a C identifier then
    the linker will automatically see PROVIDE two symbols: __start_SECNAME
    and __end_SECNAME, where SECNAME is the name of the section.  These
    indicate the start address and end address of the orphaned section
    respectively.

Systems that don't support these features retain the earlier behavior.

This commit also fixes the annoyance that files that include coverage
counters must be listed on COVERAGE_FILES in lib/automake.mk.

This commit also fixes the annoyance that modifying any source file that
includes a coverage counter caused all programs that link against
libopenvswitch.a to relink, even programs that the source file was not
linked into.  For example, modifying ofproto/ofproto.c (which includes
coverage counters) caused tests/test-aes128 to relink, even though
test-aes128 does not link again ofproto.o.