Replace hardwired strings at many places with new macros from config.h
and use the new strusage macro replacement feature.
* common/asshelp.c (lock_spawning) [W32]: Change the names of the spawn
sentinels.
* agent/command.c (cmd_import_key): Use asprintf to create the prompt.
We better do this once and for all instead of cluttering all future
commits with diffs of trailing white spaces. In the majority of cases
blank or single lines are affected and thus this change won't disturb
a git blame too much. For future commits the pre-commit scripts
checks that this won't happen again.