By avoiding integer overflow in the implementation entirely. The
multiplication was already explicitly checked for overflow, so also
add a check for the addition and remove the overflow checks after
the calculation.

This patch adds missing newlines, trims multiple redundant final
newlines into a single one, and trims redundant leading newlines.

According to POSIX, a line is a sequence of zero or more non-' <newline>'
characters plus a terminating '<newline>' character. [1] Files should
normally have at least one final newline character.

C89 [2] and later standards [3] mention a final newline:
"A source file that is not empty shall end in a new-line character,
which shall not be immediately preceded by a backslash character."

Although it is not mandatory for all files to have a final newline
fixed, a more consistent and homogeneous approach brings less of commit
differences issues and a better development experience in certain text
editors and IDEs.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_206
[2] https://port70.net/~nsz/c/c89/c89-draft.html#2.1.1.2
[3] https://port70.net/~nsz/c/c99/n1256.html#5.1.1.2

* PHP-7.1:
  Fixed bug #46781 (BC math handles minus zero incorrectly)

* PHP-7.0:
  Fixed bug #46781 (BC math handles minus zero incorrectly)

Actually, there is no negative zero at all.  We obey Postel's law, and
still accept negative zeroes, but we store them as positive zeroes
after the conversion from string, i.e. we normalize before further
processing.

* PHP-7.1:
  Fixed bug #75178 (bcpowmod() misbehaves for non-integer base or modulus)

* PHP-7.0:
  Fixed bug #75178 (bcpowmod() misbehaves for non-integer base or modulus)

Since `bcpowmod()` does not support non-integral operands, we have to
truncate these in addition to emitting a respective warning. We also
have to work with the truncated values in the following.

We recognize that the division by one to enforce the truncation is
actually overkill, but we stick with it for now, and shall tackle the
issue for PHP 7.3.

* PHP-7.1:
  Fixed bug #54598 (bcpowmod() may return 1 if modulus is 1)