Share code on to_xfer_partial for tfile and ctf target

Share code on to_xfer_partial for tfile and ctf target

In the to_xfer_partial implementations of ctf and tfile, the code on
reading from read-only sections is duplicated.  This patch moves it to
a separate function exec_read_partial_read_only.

gdb:

2014-02-23  Yao Qi  <yao@codesourcery.com>

	* ctf.c (ctf_xfer_partial): Move code to ...
	* exec.c (exec_read_partial_read_only): ... it.  New function.
	* tracefile-tfile.c (tfile_xfer_partial): Likewise.
	* tracefile.c: Include "exec.h".
	* exec.h (exec_read_partial_read_only): Declare.