Use function.line+offset to locate breakpoints

[Andrew Cagney]

[suggested by RMS]

Say the user enters:

    (gdb) break file.c:100

GDB stores this as {file="file.c",line==100} and then maps that onto a 
breakpoint address.

When the program is restarted, GDB re-uses the same technique.  Again 
setting the breakpoint at {file="file.c",line==100} converted into an 
address.  Unfortunatly, if the source file changes, that location is no 
longer correct.

RMS suggests that we, instead or in addition, store the line number of 
the function containing {file,line}.

By doing that we can re-insert the breakpoint at:

   new-bp-line = new-function-line
                 + (old-bp-line - old-function-line)

i.e., a known offset into the function.

Doing this will make GDB robust to at least the most basic of source 
code changes.

Thoughts?
Andrew