gdb.base/foll-fork.exp: refactor to restart GDB between each portion of the test

This test is difficult to follow and modify because the state of GDB is
preserved some tests.  Add a setup proc, which starts a new GDB and runs
to main, and use it in all test procs.  Use proc_with_prefix to avoid
duplicates.

The check_fork_catchpoints proc also seems used to check for follow-fork
support by checking if catchpoints are supported.  If they are not, it
uses "return -code return", which makes its caller return.  I find this
unnecessary complex, versus just returning a boolean.  Modify it to do
so.

Change-Id: I23e62b204286c5e9c5c86d2727f7d33fb126ed08