bsdinstall: Replace correct, but fragile, string builder with open_memstream.

The old one triggered a false positive -Warray-bounds from GCC (the
compiler assumed len was always 0), but it was also fragile with
manually computed lengths paired with strcat vs using a string
builder.

Differential Revision: https://reviews.freebsd.org/D40658

(cherry picked from commit f66a8328c3effcb4fbd7807b798d0288b865421d)