Implement breakpoint_find_if

Implement breakpoint_find_if

This commit implements the 'breakpoint_find_if' function, which allows
code external to gdb/breakpoint.c to iterate through the list of
'struct breakpoint *'.  This is needed in order to create the
'gdb/break-catch-syscall.c' file, because one of its functions
(catching_syscall_number) needs to do this iteration.

My first thought was to share the ALL_BREAKPOINTS* macros on
gdb/breakpoint.h, but they use a global variable local to
gdb/breakpoint.c, and I did not want to share that variable.  So, in
order to keep the minimal separation, I decided to implement this
way of iterating through the existing 'struct breakpoint *'.

This function was based on BFD's bfd_sections_find_if.  If the
user-provided function returns 0, the iteration proceeds.  Otherwise,
the iteration stops and the function returns the 'struct breakpoint *'
that is being processed.  This means that the return value of this
function can be either NULL or a pointer to a 'struct breakpoint'.

gdb/ChangeLog:
2015-03-11  Sergio Durigan Junior  <sergiodj@redhat.com>

	* breakpoint.c (breakpoint_find_if): New function.
	* breakpoint.h (breakpoint_find_if): New prototype.