unifdefall: optimise the loop that builds the unifdef command.

The old code used a shell loop to convert each controlling macro
definition into a command-line argument, reading the macro definitions
file each time. The new code converts the list of controlling macros
into a sed script which can run through the list of macro definitions
in one go.

Add some explanatory comments, since the code is quite meta.

Use {} instead of () for redirecting a group of commands.

Submitted by: Jonathan Nieder <jrnieder@gmail.com>

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