[gdb/tui] Fix tui compact-source

[gdb/tui] Fix tui compact-source

Consider a hello.c, with less than 10 lines:
...
$ wc -l hello.c
8 hello.c
...
and compiled with -g into an a.out.

With compact-source off:
...
$ gdb -q a.out \
    -ex "set tui border-kind ascii" \
    -ex "maint set tui-left-margin-verbose on" \
    -ex "set tui compact-source off" \
    -ex "tui enable"
...
we get:
...
+-./data/hello.c-----------------------+
|___000005_{                           |
|___000006_  printf ("hello\n");       |
|___000007_  return 0;                 |
|___000008_}                           |
|___000009_                            |
|___000010_                            |
|___000011_                            |
...
but with compact-source on:
...
+-./data/hello.c-----------------------+
|___5{                                 |
|___6  printf ("hello\n");             |
|___7  return 0;                       |
|___8}                                 |
|___9                                  |
|___1                                  |
|___1                                  |
...

There are a couple of problems with compact-source.

First of all the documentation mentions:
...
The default display uses more space for line numbers and starts the
source text at the next tab stop; the compact display uses only as
much space as is needed for the line numbers in the current file, and
only a single space to separate the line numbers from the source.
...

The bit about the default display and the next tab stop looks incorrect.  The
source doesn't start at a tab stop, instead it uses a single space to separate
the line numbers from the source.

Then the documentation mentions that there's single space in the compact
display, but evidently that's missing.

Then there's the fact that the line numbers "10" and "11" are both abbreviated
to "1" in the compact case.

The abbreviation is due to allocating space for <lines in source>, which is
8 for this example, and takes a single digit.  The line numbers though
continue past the end of the file, so fix this by allocating space for
max (<lines in source>, <last line in window>), which in this example takes 2
digits.

The missing space is due to some confusion about what the "1" here in
tui_source_window::set_contents represent:
...
      double l = log10 ((double) offsets->size ());
      m_digits = 1 + (int) l;
...

It could be the trailing space that's mentioned in tui-source.h:
...
  /* How many digits to use when formatting the line number.  This
     includes the trailing space.  */
  int m_digits;
...

Then again, it could be part of the calculation for the number of digits
needed for printing.  With this minimal example:
...
int main () {
  for (int i = 8; i <= 11; ++i) {
    double l = log10 ((double) i);
    printf ("%d %d\n", i, (int)l);
  }
  return 0;
}
...
we get:
...
$ ./a.out
8 0
9 0
10 1
11 1
...
which shows that the number of digits needed for printing i is
"1 + (int)log10 ((double) i)".

Fix this by introducing named variables needed_digits and trailing_space, each
adding 1.

With the fixes, we get for compact-source on:
...
+-./data/hello.c-----------------------+
|___05_{                               |
|___06_  printf ("hello\n");           |
|___07_  return 0;                     |
|___08_}                               |
|___09_                                |
|___10_                                |
|___11_                                |
|...

Also fix the documentation and help text to actually match effect of
compact-source.

Tested on x86_64-linux.