Check if the environment variable NCDU_SHELL is defined before the SHELL
variable is checked. This makes it possible to specify a program to
execute when 'b' is pressed. Setting SHELL to for example "mc" (Midnight
Commander) didn't work because mc already uses SHELL to execute
commands.
The check for the system() exit status is slightly problematic, because
bash returns the status code of the last command it executed. I've set
it to only check for status code 127 now (command not found) in order to
at least provide a message when the $SHELL command can't be found. This
error can still be triggered when executing a nonexistant command within
the shell and then exiting.
Key 'b' in the browse window spawns a shell in the current directoy.
We first check the $SHELL environment variable of the user for the preferred
shell interpreter. If it's not set, we fall back to the compile time
configured default shell (usually /bin/bash).
Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
