Fix bug #9193: Spacing modification not exact

The conversion from floating point to string performed by
boost:lexical_cast does not allow specifying a precision and,
for example, values such as 0.9 are resturned as 0.899999976.
The standard C++ way for performing the conversion is using
std::ostringstream which is exempt from this problem, even if
less efficient. For the sake of accuracy, boost::lexical_cast
is ditched in favor of the ostrinsgstream implementation.
In C++11 another option would be using std::to_string, but I
think it is not as efficient as the boost way and not worth
implementing through #ifdef's.
Incidentally, this patch would have also fixed #9190 and all
similar cases involving the use of convert<string>(float|double).
Revert 6a5aa1ca

That commit is obsoleted by 5bd14af8.
Use getent instead of grepping /etc/passwd.

Forthcoming versions of cygwin will use a different mechanism for
obtaining passwd/group information based on /etc/nsswitch.conf.
Thus, it will not be guaranteed that the files /etc/passwd and
/etc/group even exist. The recommended way for obtaining those
info is by using the getent command, which already works in
current versions.
Add some postscript and pdf viewers for Unix and Windows.

For Windows: AcroRd32 and gsview (both 32 and 64 bit versions).
For Unix: qpdfview.

Qpdfview is a nice alternative to Okular for KDE users and a superior
alternative to Evince for Gnome users, due to its complete synctex
support. It only depends on Qt libraries for the graphical interface.
Add forward search command for qpdfview.

Also update docs explaining how to activate reverse search in qpdfview.