textual: fix some typos and inconsistencies in various messages

Fixing plain typos, miswordings, inconsistent periods, some missing
angular brackets, and a proper pluralization (even when it involves
a constant, because for some languages the precise value matters).

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
build-sys: fix switch_root/pivot_root cut-n-paste error in configure.ac

The "linux only" check for pivot_root seems to suffer from
a cut-n-paste problem from the earlier switch_root part.

Signed-off-by: Andreas Henriksson <andreas@fatal.se>