Issue reproduction steps:

1. Spawn a GUI program e.g. open a PDF file in zathura or evince.
2. Without quitting `nnn` close the terminal.
3. Notice that the application quits too.

Can be a nagging issue if someone is not using a drop-down terminal.

Fix: detach a GUI child and start it in a new session.

There are 2 aspects to this commit:

- It fixes #81: in cases where we do not wait for a spawned child we can assume
that the child is a GUI process. We detach and spawn the child in a new session.
- It changes the behaviour to @Rahi374's suggestion in PR #40 i.e. invoke the DE
opener in F_NOWAIT mode.

This reverts commit 7be0726164.

This reverts commit 7654a2e0c6.

This reverts commit 30fdb6ff11.

Sizes like 1.999 get rounded to (1 + 100/100), which should be displayed
as 2.00 not 1.100 as simple %i.%02i would do without additional measures.

Use distinct (by username) tmp filename to copy file paths.

The pattern used is:
/tmp/nnncp$USER

If username is 'arun', the file name is `/tmp/nnncparun`.

In nav-as-you-type mode only control keys are understood when filter is enabled.
We need some way to quicly jump HOME, last visited dir or start dir.

The following pattern passes the current check:

src  - 1010
dst  - 0101
mask - 1111

This fixes the build with the BSD install command by eschewing the GNU 
specific -t option, which is not portable.