refactor(installer): variablize fogprogramdir through installer and derive PHP constants from it

[mastacontrola]

Background

The FOG base path /opt/fog is currently hardcoded in multiple places:

• lib/common/config.sh sets fogprogramdir="/opt/fog" but this doesn't flow into PHP
• packages/web/lib/fog/system.class.php defines FOG_BASE_DIR and FOG_CACHE_DIR
  as hardcoded strings (introduced in perf(settings): add TTL-based per-process cache to getSetting() with cross-process flush signal #849)
• Various functions.sh blocks reference /opt/fog/service, /opt/fog/log,
  /opt/fog/cache etc. as literal strings rather than using $fogprogramdir

Problem

Non-standard installs (different base path) require manual edits across multiple files.
There is no single source of truth for the FOG base directory that both the shell installer
and the PHP runtime share.

Proposed Solution

• Audit all hardcoded /opt/fog references in lib/common/functions.sh and replace with
  $fogprogramdir (already defined in config.sh)
• Have the installer write fogprogramdir into a PHP-readable location — either the
  generated service/etc/config.php or a new packages/web/commons/fogpaths.php — as a
  define() so the PHP runtime knows the real base path
• Update system.class.php to derive FOG_BASE_DIR from that generated define rather
  than hardcoding it, with /opt/fog as fallback if not defined
• All other path constants (FOG_CACHE_DIR etc.) derive from FOG_BASE_DIR

Notes

• High blast radius — touches installer, service setup, symlink creation, log paths,
  tftp setup, and PHP boot chain. Needs careful testing across all supported distros.
• This is explicitly out of scope for perf(settings): add TTL-based per-process cache to getSetting() with cross-process flush signal #849 which hardcoded the path
  to keep that PR focused and reviewable.
• The install validation repo (fogproject-install-validation) should catch regressions
  if tests are run after changes here.

Acceptance Criteria

• fogprogramdir change in .fogsettings is fully respected by installer
• PHP runtime derives FOG_BASE_DIR from installer-written value, not hardcoded string
• Fresh install and upgrade paths both work on all supported distros
• No hardcoded /opt/fog strings remain in functions.sh

[mastacontrola]

Prerequisite landed: #849 is implemented on working-1.6 in 86b26c2.

The hardcoded constants this issue will variablize now exist in packages/web/lib/fog/system.class.php::__construct():

define('FOG_BASE_DIR', '/opt/fog');
define('FOG_CACHE_DIR', FOG_BASE_DIR . DS . 'cache');

with an inline comment pointing here. lib/common/functions.sh also now creates /opt/fog/cache as a literal path inside installFOGServices — that string is one of the /opt/fog references this issue should derive from $fogprogramdir.