The code was quite fragile.  As a first pass, use buffers of size
PATH_MAX and LINE_MAX accordingly until we simplify the overall logic.

Avoids weird things like /etc/.. when displaying cwd.

Also no need for cwd to be on the heap.

Specifying commands by name in config.h means that
every time one switches ones editor (for example),
one has to modify every config.h file referencing that editor
and then recompile every corresponding program.

This change adds a string `env` for specifying an environment
variable to `struct key` and uses it to optionally specify the
command to run on SEL_{RUN,RUNARG}.
The `run` string is used as a fallback when the environment variable
has not been specified or is not set.
It also updates `config.def.h` to demonstrate this new capability.

Thanks Hiltjo!

Disable timeout when in filter or type mode.

Use getnstr() and be done with it.  It will even beep now!

This reverts commit 1e18b85e56.

Revert until remaining issues are fixed.  The filter search expires
after 1 second of inactivity without the revert.

Use 'q' to quit.

Delay displaying the message until return is pressed.
Also contain all printwarn() calls inside browse().

Fix some mandoc -Tlint warnings as well.

We care about the directory we are going to, so there is no reason
to get locked inside there if the parent is readable.