bug#21777: 25.0.50; gud-gdb uses a pager, which is harmful inside emacs

[Dima Kogan]

Hi. A regression occurred since 24.4.1, and gud-gdb now uses the pager
by default. This means that when gdb wants to output more than N lines,
it says

   ---Type <return> to continue, or q <return> to quit---

This is intended for gdb running in the console, but makes using gdb
from emacs much less pleasant since extra user interaction becomes
necessary. At worst, gdb sessions meant to be non-interactive (ones that
have self-continuing breakpoint commands for instance) become forcefully
interactive.


Recipe:

1. emacs -Q
2. M-x gud-gdb (select any executable; it doesn't matter)
3. gdb command: show height

Emacs 24.4.1 says

    (gdb) show height
    Number of lines gdb thinks are in a page is unlimited.

This is good. The pager is off, and emacs will receive all gdb output
without extra user interaction.

Emacs from git says

    (gdb) show height
    Number of lines gdb thinks are in a page is 24.

This is bad. After 24 lines of output gdb will pester the user. I
haven't attempted to do any debugging here yet. Probably will look at it
eventually, but if somebody knows what's wrong immediately, that'd be
great