coding style: Change the comment style in argument.c

 Instead of having two practically-useless lines, one with "/*" at
the start of the block comment, and one with "*/" at the end of the
block comment, just have one at the end, and immediately start the
block comment at the opening line.

 This makes it more readable and less inefficient, especially for
one-line comments (which will get changed in the next commit).
coding style: Change one-line comment style in argument.c

 For one-line comments, still use block comment syntax, but don't
include the extra line at the bottom to close the comment; do the
entire comment on one line.
coding style: Change comment styles used in database.{c,h}

 Follows similar style to 665df80 and fe4ad27, but also includes
the use of one-liners at the end of the line (which also get the
block-comment-style comment syntax)
coding style: Fix comment style in the rest of the project files.