Prefer xpalloc to doubling buffers by hand

Prefer xpalloc to doubling buffers by hand

* src/lread.c (grow_read_buffer): New function, which uses xpalloc.
(read1): Use it for simplicity.
* src/macros.c (store_kbd_macro_char):
* src/minibuf.c (read_minibuf_noninteractive):
* src/term.c (encode_terminal_code):
* src/xrdb.c (magic_db):
Prefer xpalloc to growing buffers by hand.
This doesn’t fix any bugs, but simplifies the code a bit.