Replace use of PATH_MAX by asprintf (C11)

[mkoeppe]

... suggested by Jerome BENOIT for Debian

[jgmbenoit]

This issue was reported to Debian: bugreport #915571 .
This actually concerns the way filenames are managed in src/util/{gensymm,genmodel,print,output).c.
The reporter suggests a quick fix that consiste to replace the strcat and strcpy calls by their n versions.
Using asprintf is certainly better.