Make linux_ptrace_attach_fail_reason return an std::string

Make linux_ptrace_attach_fail_reason return an std::string

This patch makes linux_ptrace_attach_fail_reason and
linux_ptrace_attach_fail_reason_string return std::string.  It also
replaces usages of struct buffer with std::string.  This allows getting
rid of a cleanup in in linux_ptrace_attach_fail_reason_string and
simplifies the code in general.

Something that looks odd to me is that in
linux_ptrace_attach_fail_reason, if the two messages are appended, there
is no separating space or \n, so the result won't be very nice.  I left
it as-is for now though.

gdb/ChangeLog:

	* nat/linux-ptrace.h (linux_ptrace_attach_fail_reason): Return
	std::string.
	(linux_ptrace_attach_fail_reason_string): Likewise.
	* nat/linux-ptrace.c (linux_ptrace_attach_fail_reason):
	Likewise.
	(linux_ptrace_attach_fail_reason_string): Likewise.
	* linux-nat.c (attach_proc_task_lwp_callback): Adjust.

gdb/gdbserver/ChangeLog:

	* linux-low.c (attach_proc_task_lwp_callback): Adjust to
	linux_ptrace_attach_fail_reason_string now returning an
	std::string.
	(linux_attach): Likewise.
	* thread-db.c (attach_thread): Likewise.