axrun: run commands with the same partial parameters

The purpose of the utility is that we can run several commands from one
command line, where we can get the commands use the same prameters
(without storing these common parameters in evironment variables).

Usage:
axrun {set var=value} cmd1 {params} + cmd2 {params} +cin cmd3 {params} ...

The 'set' parameter is used to specifiy variables that are used in the
subsequent commands, in the form of '$(VAR)'. Here we can use the
environment variables (like '$(PATH)') and other, axrun-specific variables:
- YEAR
- MON
- DAY
- HOUR
- MIN
- SEC
These variables contains the value of starting time of the command line.

The subsequent commands are separated either by '+' or '+cin'. The second
one indicates that the following command receives the input of the
axrun (its standard input).
We can use the varibales (that were defined with 'set') in these commands.

Examples:
axrun set APPLE=apple ls '$(APPLE)' + cp '$(APPLE) $(APPLE).bak'
  -> the 'apple' directory will be listed and copied into a temp dir

afind '*.txt' | axrun set 'NAME=$(YEAR)_$(MON)_$(DAY).zip' +cin azip -stdin
-o '$(NAME)' + cp '$(NAME)' bak + scp '$(NAME)' user1@10.20.30.50:/home/archiv
  -> find the text files, zip them into a file named after the current
     time, and copy the zip into the archive directory and an other archive
     directory a remote machine (the zip out and the copy in file names are
     the same)

Note the apostrophes around the variable names in the examples: they are
necessary to avoid the shell processing of the variable names (they don't
exist in the shell environment). These are necessary on Unices, the Windows
command line doesn't need them.