vimrc: ignore trailing whitespace on old Vim.

Vim before 7.1.40 doesn't have the match functions, so disable this where it's
not present.  Otherwise editing on CentOS 5 becomes very painful.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
zsh: add a function to produce colors portably.

The color_fg function takes into account older systems, such as CentOS 5, that
lack support for the new color primitives in zsh 4.3.9 and newer.  It also
supports 256 colors on all systems that have zsh 4.3.9+ or tput.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
prompt: use color_fg.

This pushes all the nasty color handling code into exactly one place.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
prompt: remove unused function.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
prompt: fix help text.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
prompt: don't try to run vcs_info on old systems.

Some old versions of zsh don't have vcs_info, so make it not complain there.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
prompt: clean up comments.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
zsh: add a color_reset function.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
zsh: modify less colors to use color_fg.

This avoids the inevitable portability issues.  Also, change the standout mode
to a blue with no background.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
prompt: ensure text displays properly in old zsh.

In old versions of zsh, the %V prompt character does not exist.  As a
consequence, the p_mode (ok, no, ◈◈) is never displayed.  Since these old
versions do not support zle-keymap-select, there is no way to ever indicate that
the prompt is in vi-cmd mode, so the best we can do is to make the "ok" or "no"
show up properly.  Modify p_mode to use a syntax that makes this possible.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>