showlinenum : Remove colon before line number

- Remove the first colon that appears before the line number unless
showing the path.

Before:
:1:+added
test1/dirfile:1:+added

After:
1:+added
test1/dirfile:1:+added

- Print the line variable using the floating point type specifier.

'line' is the line number and is an integer but Awk stores all integers
internally as floating point. Prior to this change line was printed
using the generic print which may print scientific notation depending
on how large the number.

- No longer concatenate the diff line before printing.

The elements that make up the diff line output
[path:]<line number:><diff line>
are now each printed separately.