[PATCH 0/4] Unify string-reading APIs

[Tom Tromey <tromey@adacore.com>]

An old comment notes that gdb has several different ways to read a
string from the target.  This series tries to simplify this situation.

read_memory_string is removed entirely.  Perhaps this is the better
name, in the end; but I didn't want to move read_string into
corelow.c.  Maybe we need a new file for the memory-reading functions
that are there?  (Also I wonder if any of the remaining ones are
redundant.)

target_read_string is first rewritten in terms of read_string.  A
subsequent patch changes its API to be simpler.

Perhaps further simplification could be done as well.  I'm open to
suggestions.

Regression tested on x86-64 Fedora 30.  I also built it using a mingw
cross, to make sure windows-nat.c still builds.

Tom