Use PS1 environment variable for prompt

Pick up the PS1 environment variable and use it as the command line
prompt.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Replace markers for username and host in prompt

Replace the special markers %n and %m with username and hostname, if
part of the PS1 environment variable used for making the prompt.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>